Creating energy from dairy cow manure

As we learned in the previous Step, cows cannot digest 100% of what they eat, so a considerable amount of nutrients and energy ends up in the manure.

Rather than considering manure a waste product, by processing it further inside anaerobic digesters, methane is created which can then be used as a source of energy; the solid fraction (digestate) that is left after the fermentation took place can then also be used to extract valuable nutrients with which to fertilise the land.

In the Netherlands, farming innovator Arjan Prinsen uses a modern fermentation system – a Combibag – to create electricity and heat from cow manure, which has proven to be a successful business model. Read this interview with Arjan to find how manure can be used to reduce the carbon footprint of a dairy farm.

How does a Combibag work?

Combibag is a very simple storage system for liquid and gas, with a high calamity storage capacity for gas, but low total manure input capacity and low labour costs. It consists of a fermentation chamber (digester) that is filled up to 80% with manure, the rest being the biogas (primarily methane) formed as the result of the fermentation. The Combibag is placed in the ground (just above ground water level) and isolated through the earth wall with extra insulation around the bag and the thermic isolation is increased further by an air blown hall placed over the Combibag.

The manure is pre-mixed with additional organic materials and then pumped into the digester every hour. The bacteria that ferments the manure needs to eat continuously, they cannot be starved! This is (as much as possible) an automated process, because we want to keep the system as labour-friendly as possible.

The biogas produced in the digester is continuously pumped to a small electromotor, to be either converted into electricity or used as heat. The electricity is used to power farm machinery and a few neighbouring houses. The heat is partly used for private heating and partly fed back into the digester to ensure a stable fermentation temperature of 38-40°C, the optimum for the fermenting bacteria to operate.

Please, tell us about the origin of the Combibag idea and the motives behind it…

I work for a company that makes a number of innovative products, while my parents own a cattle farm. I designed the Combibag really as a hobby; I enjoy this kind of challenge. The design is not patented and I don’t really intend to do so. I am too busy for turning it into a business! The major drive behind the creation of the Combibag was that there were issues with earlier models of digesters that were fed a mix of manure and other biomass (like maize). In those models, back in 2004, the efficiency was ultimately low because they kept on jamming, and the emission of sulphuric compounds into the atmosphere was also notable.

What hurdles/obstacles did you encounter in the implementation of the idea?

The main hurdles when it comes to most innovations are the current regulations concerning energy policies, safety regulations etc. The current version of the Combibag is also still under further development, there are improvements being made all the time. On another farm they built it in a round rather than oblong shape, to allow for more efficient mixing of the manure during the fermentation process; they also built a cement well under the ground – it is there that the fermentation takes place.

Making sure that the system is cost-effective is also very important in times when the electricity prices are descending due to new cleaner energy sources filling the market.

Which enabling elements (policy, finance, infrastructure) do you think are essential for a farmer to be able to follow your example on their farm?

Well, the regulations should allow some room for more pilot-projects to take place, so that the existing technology can be improved. This is especially true for smaller farms that would need to pool manure together in order to have a cost-effective solution; right now the hassle of analysing and transporting manure from one farm to the other, just requires too much labour to be a viable option.

Of course, the farmer who wants to adopt Combibag, or any other manure digester, must be handy with technological appliances. Those who are used to a robotic milking system, for instance, would have no problem adopting a manure digesting system; but if a farmer has no feeling for technics, then it is better not to dive into this! The main factor that makes such a system viable is the new climate policy, which rewards farmers who reduce their emissions.

For whom do you think it is a worthwhile strategy to adopt? (farm size, geographic location, practice type, barn type…?)

As mentioned, the farmer must have some affinity for technics. On top of that, the minimum farm size should be 200 cattle. Below that it simply isn’t cost-effective, and probably there isn’t enough labour force on the farm.

What do you think makes Combibag unique or superior compared to other manure digesters?

It is definitely its simplicity and ease of use. It requires minimum labour, as farmers are already very busy. It can also withstand small shocks in terms of input and temperature, because the density of the manure in the digester isn’t very high.

How do you foresee the Combibag concept will develop in the coming 5 years? Do you have plans to make a business out of your concept? Will others be able to adopt it and if so under which licencing conditions?

As I said, it is not my intention to patent the design; others are already adapting it with some variations. What is happening is that we are improving some aspects, such as the mixing, and thereby the efficiency.

We are also working at models where the different fractions are separated, so that more of the nutrients from the digestate can be used as fertilisers and that there are less emissions into the atmosphere. This is especially true for sulphates. With the new incentives that FrieslandCampina [a Dutch dairy coorperative] will be giving to its farmers, I foresee that many more will adopt a manure digester. For each m3 of manure that is processed in such a way, 10 eurocents will be rewarded to the farmer.